The present invention relates to a method and a system for maintaining the intake vacuum of an internal combustion engine at a predetermined reference value when the engine is decelerated.
Under decelerating condition of an internal combustion engine, air supply to the engine is insufficient to maintain air-fuel ratio at the stoichiometric point of the mixture, thereby resulting in the emission of high content hydrocarbon.
Various devices have hitherto been developed to mitigate this problem. These include throttle opener, throttle positioner and boost controlled deceleration device (BCDD). The latter has a valve controlled throttle bypass air passage which admits auxiliary air into the manifold at a point downstream of the closed throttle.
For purposes of reducing the undesirable emission, it is desirable that the intake vacuum be relatively low during the initial period of deceleration. Conversely, during the subsequent period of deceleration a relatively high value of intake vacuum is desirable to operate the engine as a brake. During the later period when the engine is idled, it is further desirable that the intake vacuum be lower than that during the initial deceleration period. It is impossible for the prior art devices to meet these requirements accurately.